Car screenwash (the blue one) and thin to the consistency of semi-skimmed milk, nothing else added. Always mix in separate tub, not in the airbrush and then pour into the airbrush cup through some gauze/mesh, I put my screenwash into a dropper bottle for ease of control. Getting the consistency right is something you'll achieve with practice (there isn't a magic ratio) and you'll learn you've got it right by tilting the tub you mix/thin in and seeing how it adheres to the edges and flows. In all seriousness get a small glass of semi-skimmed milk and compare
If you're using water then try and get some RO/distilled water as it will contain less minerals than tap water.
I generally spray 5 ~ 10 psi without any problems.